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PHYSICS 231 INTRODUCTORY PHYSICS I

PHYSICS 231 INTRODUCTORY PHYSICS I. Lecture 17. Recap - Heat Transfer. Heat: Q = Energy transferred due to microscopic contact Heat can: Change temperature c = specific heat For water: c= 1.0 cal/(g°C) Change state of matter L = Latent heat of fusion or vaporization

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PHYSICS 231 INTRODUCTORY PHYSICS I

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  1. PHYSICS 231INTRODUCTORY PHYSICS I Lecture 17

  2. Recap - Heat Transfer • Heat: Q = Energy transferred due to microscopic contact • Heat can: • Change temperature • c = specific heat • For water: c= 1.0 cal/(g°C) • Change state of matter • L = Latent heat of fusion or vaporization • For water: LF=79.7 cal/g, LV=540 cal/g

  3. Example 11.11 A 50 g ice cube at 0ºC is put into a styrofoam cup containing 300 g of coffee at 90ºC. Assuming no heat is lost to the cup or the air, what is the final temperature of the coffee after the ice melts? T = 65.8°C

  4. Recap - Kinds of Heat Transfer • Conduction • Hot and cold objects in physical contact • Examples: Heating a skillet, losing heat through the walls • Convection • Hot objects move (gas or liquid) • Examples: Hot-water heating for buildings Circulating air Unstable atmospheres • Radiation • Energy transferred by light (UV, IR,…) • Examples: Stars, Incandescent bulbs

  5. Conduction • Rate of heat transfer • Conductivity k is property of material • R-value also depends on thickness. It adds for layered objects (R=R1+R2)

  6. Example 11.12 What is the ratio of heat transfer for a single pane of glass (1 cm thick) to that of a double pane of glass (each 0.5 cm thick with 1 mm air between)? DATA: kglass= 0.84 W/mºC, kair= 0.0234 W/m ºC P1/P2 = 4.59

  7. Convection • Due to movement of hot gas or liquid • Hot air rises from radiator causing air currents • Air trapped between glass panes cannot transfer heat by convection, only conduction.

  8. Transfer of heat by radiation • All objects emit light if T > 0 • Colder objects emit longer wavelengths (red or infra-red) • Hotter objects emit shorter wavelengths(blue or ultraviolet) • Stefan’s Law give power of emitted radiation Emissivity, 0 < e < 1, usually near 1 s = 5.6696x10-8 W/(m2ºK4) is the Stefan-Boltzmann constant T must be in Kelvin !!!

  9. Example 11.8 If the temperature of the Sun fell 5%, and the radius shrank 10%, what would be the percentage change of the Sun’s power output? - 34%

  10. Example 11.9 DATA: The sun radiates 3.74x1026 W Distance from Sun to Earth = 1.5x1011 m Radius of Earth = 6.36x106 m • What is the intensity (power/m2) of sunlight when it reaches Earth? • How much power is absorbed by Earth in sunlight? (assume that none of the sunlight is reflected) • What average temperature would allow Earth to radiate an amount of power equal to the amount of sun power absorbed? a) 1323 W/m2 b) 1.68x1017 W c) T = 276 K = 3 ºC = 37 ºF

  11. What is neglected in estimate? • Earth is not at one single temperature • Some of Sun’s energy is reflected • Reduces TE ~ 20°K • Emissivity lower at Earth’s thermal wavelengths than at Sun’s wavelengths (due to atmosphere) • Increases TE ~ 40°K • Natural greenhouse effect - necessary for life on Earth • Radioactive decays inside Earth are additional • source of energy • Small effect for Earth • Jupiter radiates much more energy than it receives from the sun

  12. Example 11.10a Two Asteroids A and B orbit the Sun at the same radius R. Asteroid B has twice the surface area of A. (Assume both asteroids absorb 100% of the sunlight and have emissivities of 1.0) The average temperature of B, TB = _____ a) (1/4)TA b) (1/2)TA c) TA d) 2TA e) 4TA

  13. Example 11.10b Two identical asteroids A and B orbit the sun. Asteroid B is located twice as far from sun as Asteroid A. RB=2RA (Assume both asteroids absorb 100% of the sunlight and have emissivities of 1.0) The average temperature of B, TB = _____ a) (1/4)TA b) (1/2)TA c) (2-1/2)TA d) (2-1/4)TA e) TA

  14. Example 11.10c Two Asteroids A and B orbit the Sun at the same radius R. Asteroid B is painted with reflective paint which reflects 3/4 of the sunlight, while asteroid A absorbs 100% of the sunlight. Both asteroids have emissivities of 1.0. The average temperature of B, TB = _____ a) (1/4)TA b) (1/2)TA c) (2-1/2)TA d) (2-1/4)TA e) (2-3/4)TA

  15. Example 11.10d Two Asteroids A and B orbit the Sun at the same radius R. Asteroid B has an emissivity of 0.25, while the emissivity of asteroid A is 1.0. Both asteroids absorb 100% of the sunlight. The average temperature of B, TB = _____ a) 4TA b) 2TA c) 21/2TA d) 21/4TA e) 23/4TA

  16. Greenhouse Gases • Sun is much hotter than Earth so sunlight has much shorter wavelengths than light radiated by Earth (infrared) • Emissivity of Earth depends on wavelength • CO2 in Earth’s atmosphere reflects in the infrared • Barely affects incoming sunlight • Reduces emissivity, e, of re-radiated heat

  17. Global warming • Tearth has risen ~ 1 ºF in past 100 years • “most of observed increase […] is very likely due to the observed increase in anthropogenic greenhouse gas concentrations” Intergovernmental Panel on Climate Change (IPCC)

  18. Mercury and Venus Tmercury = 700 K (day) & 90 K (night) Tvenus = 740 K

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